1. Technical Field
The disclosure relates to apparatus for controlling a modulation signal for use in modulating the power supply of a power amplifier, a module comprising the apparatus, a transmitter comprising the module, and communications equipment comprising the transmitter.
2. Description of the Related Art
Polar modulation architecture has become quite popular in recent years, at least in scientific publications, as an alternative to the direct Cartesian (I&Q) up-conversion architecture, for transmitters targeting hand-set applications where communication standards with amplitude modulation, such as EDGE, are used. This is because a polar modulation transmitter can have a lower power consumption and lower cost. The cost is lower, at least in part, because there is no need for a surface acoustic wave (SAW) filter at the output of the transmitter.
A typical polar modulation transmitter 100 of the prior art is illustrated in FIG. 1. Separate amplitude and frequency components of the modulation signal are generated in the digital domain and provided to, respectively, inputs 110 and 120. A sigma-delta converter 140 converts the digital frequency component to a digital control signal for controlling the divider ratio of a fractional-N phase locked loop 150 for modulating the phase of a voltage controlled oscillator 155. The output of the voltage controlled oscillator 155 is coupled to a saturated power amplifier (PA) 160, and the amplified signal is provided at an output 130 to an antenna. The digital amplitude component is provided to an amplitude modulation controller 170 which converts the digital amplitude component to the analog domain and scales it based on a scaling signal at an input 175 to provide an analog amplitude modulation signal to a power supply modulator 180 for modulating the supply voltage of the PA 160. This supply voltage determines the power of the amplified signal output by the PA 160 and hence to the antenna. Typically, the power supply modulator 180 and the PA 160 are provided in a front-end module (FEM) 190.
Typically, the PA 160 needs a certain DC voltage to start providing any signal at the output 130 to the antenna. This is illustrated in FIG. 2 by a typical graph of PA output amplitude as a function of PA supply voltage, where it can be seen that almost no output power is generated until the supply voltage reaches about 0.2V. So, the amplitude modulation controller 170 usually receives a DC offset control signal at an input 185 for controlling DC offset in the analog amplitude modulation signal.
The desired power of the signal at the antenna depends on the length and quality of the signal path between the transmitter 100 (in a handset) and a base station. These parameters can vary greatly, meaning that the PA 160 preferably provides the amplified signal over a large range of power levels, typically about three power decades.
If the scaling is performed in the digital domain, the amplitude modulation controller 170 usually requires a digital-to-analog converter (DAC) having a high dynamic range in order to provide an analog amplitude modulation signal having a good signal-to-quantization-noise ratio and suitable for allowing the power supply modulator 180 to modulate the supply voltage to the PA 160 sufficiently for the PA 160 to output the amplified signal over the desired large range of power levels. However, a typical high dynamic range DAC having over 14-bits and using a sampling frequency of at least 3.25 MHz requires a large operating current and a large chip area in an integrated circuit. Likewise, if the scaling is performed in the analog domain, prior art devices employ complex analog circuit elements to obtain fine control of the gain applied to the analog signal over a wide range of gain values.